The membrane bound enzyme Na,K-ATPase (EC 3.6.1.3) is responsible for the active transport of sodium and potassium across the cell membrane. Cardiac glycosides bind to and inhibit the enzyme. Cardiac glycosides are widely used clinically in the treatment of congestive heart failure, and it now seems certain that the therapeutic effect of cardiac glycosides is manifested via binding to the Na,K-ATPase; i.e., Na,K-ATPase is the pharmacologic receptor for cardiac glycosides. To understand how cardiac glycosides produce their therapeutic effect, the molecular structure of the receptor must be determined. Due to the lack of x-ray crystallographic and amino acid sequence data any molecular model, which is essential to the understanding of the mechanism, must of necessity be speculative. The research proposed here will identify, localize and characterize the digitalis receptor site, the ATP binding site and the sites which regulate these sites. The important amino acid residues within these sites, and their role in the mechanism of interaction of digitalis with the Na,K-ATPase will be delineated. This structural information is essential to the elucidation of the reaction mechanism of the Na,K-ATPase and to the mechanism of digitalis interaction with its receptor.